This invention relates to a method of monitoring the state of charge of a rechargeable battery and, more particularly, a nickel-cadmium battery, said method comprising the storage of a reference value corresponding to a certain state of charge, the measurement of terminal voltage, current and time during each battery charging and discharging cycle and the conversion of the quantities so measured into quantities corresponding to the energy charged or discharged, accounting for the direction of current flow. In addition, the invention relates to an apparatus for carrying out said method. The term "rechargeable battery" as used herin refers to monocells as well as any combination comprising a plurality of cells.
Rechargeable batteries have become increasingly important mainly to supply emergency power to computing facilities, to control systems for machinery and equipment and to medical apparatuses, to supply electricity to measuring, signalling and alarm devices and to provide power for traction or to start agricultural machinery as well as aeronautical equipment and equipment for space travel. Continuous monitoring of the state of charge of such batteries by means which are as accurate as possible is necessary for users in many applications to avoid a stoppage or an interruption of the operation of the equipment served by such batteries, such as a computer or a measurement or control or alarm system. Existing battery cell capacities can only be fully utilized and charging or discharging beyond certain limits, having detrimental effects upon batteries, can only be avoided by state-of-charge monitoring.
Devices for monitoring the state of charge of nickel-cadmium batteries heretofore known usually employ one of two different methods.
According to the more simple one of said two methods, cell terminal voltage is measured, an upper voltage limit defining the end of the cell charging cycle and a lower voltage limit defining the end of the cell discharging cycle. The accuracy of the use of terminal voltage as a measure of the electrochemical cell conditions may be improved by correcting the terminal voltage so measured as a function of the charging or discharging currents and cell temperature measured together with said terminal voltage.
It is an innate defect of the above method that the application of said method will only provide a signal indicating that the battery is fully charged, the charging current must be switched off and full charge must now be maintained by appropriate means as well as a signal indicating that the battery is empty and the load must be switched off.
The second method heretofore known employs the coulometry principle. Said method calls for the measurement of the energy delivered by the battery being in a predetermined initial state of charge which is normally the fully charged condition. According to said method, signals corresponding to the quantity of energy delivered to or by the battery are transmitted to a counter or a reference cell and a signal representing the remaining battery charge available for delivery is obtained from the difference between the design cell capacity and the energy delivered by the battery.
As in the case of the first method referred to hereinabove, the remaining charge so obtained may be corrected as a function of current, cell temperature and charging efficiency for indication.
Devices are even known which measure the number of battery charging and discharging cycles and battery life to predict the effect of battery ageing or spontaneous cell discharging on the design capacity of the battery.
It is a well-known disadvantage of the second method described that the state of charge determined by continuous ampere-hour measurement may, in the case of batteries operated in continuous cycles, differ from the actual state of charge, because real battery behavior and hence actual cell capacity cannot be predicted sufficiently accurately in spite of corrections made as a function of different quantities that may be measured. The coulometry method disclosed by the West German patent application No. 30 12 356 therefore proposes the elimination of this source of error by manual resetting after each completed charging cycle to compare the state of charge predicted from the measured energy input and output and the actual state of charge.